1. Field of the Invention
This invention relates to a method of and an apparatus for reading color image on an original, and particularly to a method and apparatus for photoelectrically reading color information on an original by employing a plurality of light sources having different properties from one another.
2. Description of the Prior Art
A method for photoelectrically reading color image, i.e., color information on an original which has recently attracted public attention comprises steps of sequentially lighting a plurality of light sources having different spectral properties synchronized with the scanning of the original to produce a plurality of picture signals with respect to each scanning line of the original, and discriminating colors of the image on the basis of the levels of these picture signals.
Such a method for reading color image will be described hereinbelow by referring to FIGS. 1 through 3. As an example, a method for producing three kinds of color picture signals of "blue", "red" and "black" by employing two fluorescent lamps of blue and red lamps is described.
Referring to FIG. 1, a blue fluorescent lamp 1 and a red fluorescent lamp 2 are lit by means of a lighting circuit (not shown) and the same portion (lines) of an original MS is illuminated by either blue light BC emitted for the blue fluorescent lamp 1 or red light RC from the red fluorescent lamp 2. The reflected light from the original MS is focused upon an image sensor 4 comprising, for example, a CCD (charge coupled device) line sensor by means of a lens 3, where it is converted into an electric signal (photoelectric conversion signal CE) having a level corresponding to a quantity of the focused light. In this case, for example, calcium tungstate is used as fluorescent substance of the blue fluorescent lamp 1, while, for example, magnesium germanate is employed as a fluorescent substance of the red fluorescent lamp 2. FIG. 2 shows spectral properties of both the blue and red fluorescent lamps 1 and 2.
In this conventional method for reading color image, the blue fluorescent lamp 1 and the red fluorescent lamp 2 are alternately lit, synchronized with the scanning of the original so that readings are made twice for the same line of the original MS each time when the blue and red fluorescent lamps 1 and 2 are lit.
FIGS. 3(a) to 3(d) are timing charts for showing driving modes of this conventional method for reading color image wherein FIG. 3(a) shows the scanning operation of the image sensor 4 as it scans the original MS FIG. 3(b) shows the generating manner of the blue light BC, i.e., the manner of turning on and off the blue fluorescent lamp 1, FIG. 3(c) shows the generating manner of the red light RC, i.e., the manner of turning on and off the red fluorescent lamp 2, and FIG. 3(d) shows photoelectric conversion signals CE outputted from the image sensor 4 in response to the reading and scanning. In FIGS. 3(a) to 3(d), the first line of the original MS consists of blue images (i.e. blue line), whilst the second line consists of red images (i.e. red line).
In the case when the blue light BC is projected during a first scanning period of time (period of time t.sub.1 to time t.sub.2) on the first line of the original MS as shown in FIG. 3(b), the electric charge corresponding to the reflected light during the above scanning period of time is stored in the image sensor 4, and the charge thus stored is outputted from the image sensor 4 as the photoelectric conversion signal CE during the following scanning period (period of time t.sub.2 to time t.sub.3) (see FIG. 3(d)). (The maximum amount of reflected light is obtained under this condition and is referred to as "100% reflected light".)
The second scanning is effected with respect to the same first line of the original MS during the period of time t.sub.2 to time t.sub.3. During this scanning period, the blue light BC is turned off and red light RC is projected in place of the blue light BC as shown in FIGS. 3(b) and 3(c). The red light RC in this case is substantially absorbed by the blue images on the first line of the original MS (see FIG. 2). Accordingly, small amount of light is reflected and therefore the photoelectric conversion signal CE outputted from the image sensor 4 during the following scanning period (period of time t.sub.3 to time t.sub.4) are small as shown in FIG. 3(d). During scanning periods of time t.sub.4 to time t.sub.5 and time t.sub.5 to time t.sub.6, the seond line (red line) is scanned. In this case, photoelectric conversion signal CE resembles the mirror image of the CE signal of the case of scanning the first line (blue line) (see FIG. 3(d)).
In the case when scanning is effected by the image sensor 4 with respect to a white line, substantially 100% reflected light can be obtained for both the blue light BC and the red light RC. In this case, the photoelectric conversion signals CE are of a high level in response to the 100% reflected light. However, in the case when scanning operations are effected with respect to a black line, both the blue light BC and the red light RC are absorbed so that the resulting photoelectric conversion signals CE are of a low level.
In the color discriminating circuit 5 in FIG. 1, level discrimination of the photoelectric conversion signals CE is carried out per picture element, and colors of an image on the original MS are successively discriminated on the basis of a combination of varied cases such as the above cases.
By adopting the above-mentioned conventional method for reading color images, a construction of the optical system becomes simple, and reading precision is elevated. However, there is the disadvantage that the reading speed is restricted due to the limited response speed of the light source in being turned on and off.
For instance, in the example as mentioned above, a fluorescent substance (e.g., magnesium germanate) for emitting red light used in the red fluorescent lamp 2 has generally inferior response characteristics as compared with those of a fluorescent substance (e.g., calcium tungstate) for emitting blue light employed in the blue fluorescent lamp 1. Therefore, the after-glow of the red fluorescent lamp (approx. 2 m sec) lasts longer than that of the blue fluorescent lamp. Accordingly, the red light RC rises slower than the blue light BC as shown in FIG. 3(c). As a result, scanning cannot be accurately effected during after glow periods of time t.sub.3 to time t.sub.4 and time t.sub.6 to t.sub.7 (shaded portions in FIG. 3(c)). To solve this problem various countermeasures have been considered heretofore such that the afterglow period is used for feeding the original MS. However, such countermeasures did not relieve the restriction on the original reading rate so that the application of such method as set forth above to high-speed machines has been considered to be impossible.